Its works . But when Rb reboot and come back it display jan/01/1970 and after that it display the correct time. I think when rebooted it lost the NTP connection and when it connect with AP it got sync with NTP server.

it is sycn with ntp server .but the problem is that when it rebooted it show the 1970 date and time, so i can't get the date and time of reboot or power failure .

Use a syslog server. That will at least show you the time and date when the RouterBoard starts back up. If power failures are a big problem then get a smart UPS with ethernet interface and log alarms from that.